Semiconductor manufacturers continue to measure and control the level of contamination in the processing environment, especially during the critical steps of the photolithography processes. The typical means of determining the quality and quantity of contamination in gas samples in cleanroom manufacturing environments involves sampling air and purge gases, such as, for example, filtered and unfiltered air, clean dry air, and nitrogen, with sampling tubes or traps, typically containing an adsorptive medium such as, the polymer Tenax(copyright). This sampling process is followed by analysis using thermal desorption, gas chromatography and mass spectrometry (TD/GC/MS). The combination of TD/GC/MS provides identification of sample components and a determination of the concentration of these components. The most abundant contaminants in these manufacturing environments are low molecular weight components such as acetone and isopropyl alcohol. The current sampling time for existing traps typically varies between 0.5 and 6 hours with total accumulated sample volumes ranging typically between 20 and 50 liters.
Further, in applications that are directed to the manufacturing of or use of optical elements such as, for example, photolithography, the detection and quantification of compounds having a higher molecular weight such as, for example, siloxanes is of primary concern. These compounds having a higher molecular weight are however, typically in much lower concentrations as compared with the low molecular weight species. Further, the compounds having a high molecular weight can also be defined as condensable compounds with a boiling point typically greater than approximately 150xc2x0 C. The current methods for determining contamination has the limitation of the sample volume being based on the total trap capacity of the lighter or lower molecular weight components, for example, compounds having typically less than six carbon atoms. As the heavier components are usually present at much lower concentrations, the collection of a significant mass of these higher molecular weight species is limited.
In addition, polluting or contaminating substances may adhere onto the optical elements and reduce the transmission of light. Currently airborne contamination is addressed in cleanroom environments with little regard for contaminants that may be adsorbed onto the surfaces of optical elements. The adsorbed contamination reduces the transmission of light through the optical elements and system.
Thus contamination of optical systems is emerging as a significant risk to photolithography as shorter wavelengths of the electromagnetic spectrum are exploited. However, molecular films on optical surfaces physically absorb and scatter incoming light. Scattered or absorbed light in photolithography optical surfaces causes distortion of the spherical quality of wavefronts. When the information contained in the spherical wavefront is distorted, the resulting image is also misformed or abberated. Image distortions, or in the case of photolithography, the inability to accurately reproduce the circuit pattern on the reticle, cause a loss of critical dimension control and process yield.
A need still exists for determining, accurately and efficiently, the presence and quantity of contaminants that can alter and degrade the optical systems in semiconductor processing instruments.
The system of the present invention provides an accurate and efficient system of determining and/or controlling the quality and/or quantity of contamination within a gas sample which can reduce the performance of optical elements used in semiconductor processing instruments, such as, for example, within the light path of a deep ultraviolet photolithography exposure tool. In a preferred embodiment of the present invention, the contamination may be gaseous as well as contamination adsorbed onto optical surfaces. Optical performance can be evaluated without limitation as the level of transmitted or reflected light through an optical system. The system and method of the present invention are predicated on the recognition that compounds having both high and low molecular weights can contribute to the contamination of optical systems but can operate at different rates. As such, the contaminants that negatively impact the performance of optical elements can be described in terms of different order, such as, for example, first, second and third order effects.
First and second order contaminating effects have a greater impact on contamination of optical systems than third or fourth order contaminants. The first order contaminants may comprise high molecular weight organics such as, for example, C6 siloxanes and C6 iodates with an inorganic component which is not volatilized through combination with oxygen. Second order contaminants may comprise high molecular weight organics, such as, for example, compounds including carbon atoms within the range of approximately six to thirty carbon atoms (C6-C30). Third order effects can arise due to the contaminating effects of organics such as C3-C6 that have approximately three to six carbon atoms. Further, fourth order contaminants include organics such as, for example, methane, that have approximately one to five carbon atoms. In many applications, the first and second order contamination can have a much lower concentration than the third and/or fourth order contamination, yet have a significantly greater effect on the operation of the system.
A preferred embodiment in accordance with the present invention of a method for detecting and monitoring, and preferably removing contamination in a semiconductor processing system includes delivering a gas sample from the processing system to a collection device. The method further includes collecting contamination which comprises refractory compounds, and high and low molecular weight compounds, from the gas in the collection device by sampling the gas for a duration exceeding the saturation capacity of the collection device for high molecular weight compounds. The compounds having a high molecular weight are condensable with a boiling point typically greater than approximately 150xc2x0 C.
A preferred embodiment of the system and method of the present invention for determining contamination includes the detection of refractory compounds such as, for example, siloxanes, silanes and iodates, and high molecular weight organics. The preferred embodiment includes the removal of refractory compounds, high molecular weight organics and low molecular weight organics, all of which contribute to the contamination of optical systems, but which can operate at different contamination rates.
The system of the present invention for determining contamination can use different types of sample collecting media. In a preferred embodiment, the sample collecting media can emulate the environment of the optical surfaces of interest such as, for example, the absorptive or reactive properties of the optical surfaces. A measure of contamination adsorbed onto optical surfaces enables the minimization and preferably the removal of the contaminants. In another preferred embodiment, a polymer that has a high capacity for absorbing the compounds with a high boiling point is used in a collection device such as, for example, Tenax(copyright) a polymer based on 2-6 diphenyl p-phenylene. The operation of the system in accordance with a preferred embodiment of the present invention includes quantitatively measuring the concentration of both low and high boiling point compounds in the same sample wherein the collection device has been driven beyond the breakthrough volume or saturation capacity of the collection media to capture the low molecular weight compounds. The breakthrough volume of the collection device is defined in a preferred embodiment as the quantity of gas needed to go beyond the adsorption capacity of the device.
In accordance with a preferred embodiment of the present invention, the method for detecting contamination includes a sampling time extended by, for example, a number of hours, days or weeks to enable collection of an appropriate mass of contaminants which are present in relatively low concentration. In a preferred embodiment, the sampling time is typically beyond the breakthrough capacity of the collection device for low molecular weight components and is at least six hours long and preferably within a range of six to twenty-four hours for a sampling tube system. The extended time allows for the collection of higher masses of refractory compounds and higher molecular weight compounds that may interfere with the performance of optical components even more than low molecular weight compounds. The higher molecular weight compounds include, but are not limited to, for example, siloxanes and silanes.
In accordance with another preferred embodiment of the present invention, a semiconductor processing instrument, for example, a photolithography cluster, includes a filtering system to remove contaminants. The filtering system includes a selective membrane to filter organic compounds from a gas stream.
The foregoing and other features and advantages of the system and method for determining and controlling contamination will be apparent from the following more particular description of preferred embodiments of the system and method as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.